Pelagic Sargassum, floating macroalgae, has been a subject of study for decades, particularly its ability to thrive in nutrient-poor open ocean waters. The emergence of the Great Atlantic Sargassum Belt (GASB) since 2011 has dramatically altered the landscape of the tropical Atlantic and Caribbean, where Sargassum was previously less abundant. This massive bloom presents significant ecological and economic consequences. While Sargassum provides habitat for various organisms, the GASB has caused widespread coastal inundations, harming nearshore ecosystems like seagrass beds and coral reefs, impacting sea turtle hatchling survival, and creating economic challenges for tourism-dependent regions. Sargassum decay also leads to respiratory and other human health problems, and the presence of arsenic in Sargassum tissue limits its potential use as biomass. Several hypotheses attempt to explain the GASB's dynamics, including anomalous wind patterns that dispersed Sargassum from the Sargasso Sea into the equatorial current system. However, the primary driver behind this phenomenon remains unresolved: the role of nutrient supply. This study focuses on investigating whether increased nutrient availability in the GASB is a key factor driving the massive Sargassum growth, analyzing Sargassum nutrient content and arsenic levels to understand this basin-scale ecological event and its implications.

Previous research has established that Sargassum populations in their native Sargasso Sea habitat are nutrient-limited, particularly in phosphorus. The enhanced nutrient availability has been posited as a key factor in the surge of Sargassum biomass observed in the GASB. Several studies have suggested potential nutrient sources, including upwelling, vertical mixing, discharge from major rivers such as the Amazon and Congo, and atmospheric deposition. However, pinpointing the specific sources and their relative contributions remains challenging. Previous work has also documented the bioaccumulation of arsenic in Sargassum, raising concerns about its toxicity and the limitations on the use of stranded biomass. Studies on the metal and metalloid concentrations in Sargassum have reported varying arsenic levels across different regions and species. Prior research has also highlighted the impacts of Sargassum blooms on coastal ecosystems, including negative effects on seagrass, coral reefs, and sea turtle populations. The economic consequences of Sargassum inundation, particularly on tourism, have also been extensively documented.

To investigate the nutrient status and arsenic biogeochemistry of Sargassum in the western Atlantic, samples were collected in spring 2021 along two hydrographic sections (GO-SHIP lines A20 and A22) intersecting the western GASB. Hydrographic profiles and water samples were collected using a CTD rosette system with Niskin bottles. Nutrient analyses were performed at sea using a Seal Analytical continuous-flow AutoAnalyzer 3, following standard GO-SHIP methods. Sargassum samples were collected using a dip net, sorted into *S. fluitans* and *S. natans* based on morphology, cleaned, dried, and powdered for elemental analysis and isotopic composition determination. Elemental analysis (C, N, P, As) was performed using established methods. Stable nitrogen isotope (δ15N) analysis provided insights into nitrogen sources. To determine the origin of Sargassum samples, passive particles were inserted into a numerical ocean model hindcast at each collection site and tracked backward in time. The relationship between arsenic and phosphorus content was analyzed, considering Michaelis-Menten nutrient uptake kinetics and existing theoretical predictions.

The study revealed three distinct regimes in Sargassum nutritional status across the sampling domain. In the Sargasso Sea, carbon content was high, while nitrogen and phosphorus content were low. In contrast, GASB Sargassum showed significantly higher nitrogen and phosphorus content, indicating a more nutrient-rich environment. Elemental ratios (C:N, C:P, N:P) reflected these trends, with higher ratios in the Sargasso Sea and lower ratios in the GASB. Particle backtracking analysis revealed diverse origins of Sargassum samples, with some originating from the southeast, east, north, or Gulf Stream. δ15N values provided insights into nitrogen sources, showing depleted values in the Sargasso Sea, Caribbean, and western tropical Atlantic, potentially reflecting nitrogen fixation or atmospheric deposition. High δ15N values in the northern Sargassum Sea and southern western tropical Atlantic suggest riverine influence, potentially from the US East Coast or Amazon River, respectively. However, elemental composition did not show corresponding anomalies in the latter. Hydrographic data indicated deeper nitracline and phosphocline in the Sargasso Sea compared to shallower depths in the GASB, consistent with the differences in Sargassum nutrient content. Arsenic content showed regional variations, being lowest in the GASB and highest in the Sargasso Sea, with the highest As:P ratios in the Sargassum Sea region where phosphorus limitation is known. The As:P ratio in Sargassum was shown to be a hyperbolic function of phosphorus content, consistent with theoretical predictions from Michaelis-Menten kinetics. This relationship demonstrates that arsenic content can be a diagnostic of phosphorus limitation in Sargassum. Additional analysis showed no hyperbolic relationship between As:C and carbon content, while As:N showed a hyperbolic dependence on nitrogen content, attributed to covariation in nitrogen and phosphorus content. Negative correlations were found between arsenic and both phosphorus and nitrogen, consistent with phosphorus stress, while a positive correlation existed between arsenic and carbon.

The findings clearly demonstrate that Sargassum in the GASB is enriched in both nitrogen and phosphorus compared to its Sargassum Sea habitat, strongly suggesting that enhanced nutrient availability is a primary driver of the GASB's formation and growth. Stable nitrogen isotope data provides some evidence for riverine sources in specific areas, but the identification of all sources remain a challenge. The spatial patterns in Sargassum nitrogen and phosphorus content can potentially serve as fingerprints for tracing the origin of nutrients supporting GASB growth. However, the strong seasonal and interannual variability of the GASB makes obtaining synoptic data challenging. The observed hyperbolic relationship between arsenic content and phosphorus content in Sargassum has significant implications. High arsenic concentrations in GASB Sargassum limit its use as biomass. If phosphorus supply to the GASB were to decrease relative to nitrogen, arsenic content would likely increase even further, posing greater management challenges, especially if the nutrient sources turn out to be anthropogenic.

This study provides the first comprehensive demonstration of the enhanced nitrogen and phosphorus status of GASB Sargassum, confirming nutrient availability as a key driver of this phenomenon. Stable nitrogen isotopes provide clues about nutrient sources, but more extensive synoptic data is needed. The arsenic content in Sargassum serves as a useful indicator of phosphorus limitation. Expanded observational and modeling efforts are crucial for understanding the GASB's complexities and developing effective mitigation strategies. Improved predictions and a thorough understanding of causal factors would greatly aid in proactive planning and management actions.

The study's sampling was limited to spring 2021 and two specific hydrographic sections, limiting the ability to generalize findings to the entire GASB and other seasons. The classification of Sargassum species relied on morphology, which might not perfectly capture genetic diversity. The use of a numerical ocean model for backtracking has inherent uncertainties related to model resolution and accuracy of the inputs used.